feat(forwarder): make checkpoint recovery idempotent

Preserve interrupted provider output, keep checkpoint projections deterministic, and enforce proto snapshot synchronization.

internal/backend/forwarder/file_store.go

@@ -690,9 +690,21 @@ func appendEntriesInPlace(conversation *ConversationFile, entries []HistoryEntry
 	}
 	now := time.Now().UTC()
 	assigned := make([]HistoryEntry, 0, len(entries))
+	existingIdempotencyKeys := make(map[string]struct{})
+	for _, existing := range conversation.Entries {
+		if key := strings.TrimSpace(existing.IdempotencyKey); key != "" {
+			existingIdempotencyKeys[key] = struct{}{}
+		}
+	}
 	maxTurnSeq := conversation.NextTurnSeq - 1
 	for _, entry := range entries {
 		next := entry
+		if key := strings.TrimSpace(next.IdempotencyKey); key != "" {
+			if _, exists := existingIdempotencyKeys[key]; exists {
+				continue
+			}
+			existingIdempotencyKeys[key] = struct{}{}
+		}
 		if next.CreatedAt.IsZero() {
 			next.CreatedAt = now
 		}

internal/backend/forwarder/interrupted_output_test.go

@@ -0,0 +1,187 @@
+package forwarder
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestAppendEntriesDeduplicatesIdempotencyKey(t *testing.T) {
+	store := NewConversationFileStore(t.TempDir())
+	entry := HistoryEntry{
+		TurnSeq:        1,
+		RequestID:      "request-1",
+		IdempotencyKey: "provider-interrupted-output:test",
+		Role:           "assistant",
+		Kind:           "assistant_text",
+		Payload:        json.RawMessage(`{"text":"partial"}`),
+	}
+
+	if _, assigned, err := store.AppendEntries("conversation-1", []HistoryEntry{entry}); err != nil {
+		t.Fatalf("first AppendEntries() error = %v", err)
+	} else if len(assigned) != 1 {
+		t.Fatalf("first AppendEntries() assigned = %d, want 1", len(assigned))
+	}
+	if _, assigned, err := store.AppendEntries("conversation-1", []HistoryEntry{entry}); err != nil {
+		t.Fatalf("duplicate AppendEntries() error = %v", err)
+	} else if len(assigned) != 0 {
+		t.Fatalf("duplicate AppendEntries() assigned = %d, want 0", len(assigned))
+	}
+
+	conversation, err := store.LoadConversation("conversation-1")
+	if err != nil {
+		t.Fatalf("LoadConversation() error = %v", err)
+	}
+	if len(conversation.Entries) != 1 {
+		t.Fatalf("persisted entries = %d, want 1", len(conversation.Entries))
+	}
+}
+
+func TestCancelPersistsInterruptedProviderOutputIdempotently(t *testing.T) {
+	service, stream, _ := testCheckpointBlobProjection(t)
+	conversation, _, _, err := service.snapshotCheckpointConversation(stream)
+	if err != nil {
+		t.Fatalf("snapshotCheckpointConversation() error = %v", err)
+	}
+	if _, err := service.store.SaveConversationWithEntries(stream.ConversationID, conversation, conversation.Entries); err != nil {
+		t.Fatalf("SaveConversationWithEntries() error = %v", err)
+	}
+
+	stream.mu.Lock()
+	stream.CurrentModelCallID = "model-call-1"
+	stream.ProviderAccumulatedText = "partial answer"
+	stream.ProviderAccumulatedReasoning = "partial reasoning"
+	stream.mu.Unlock()
+
+	cancel := InboundIntent{
+		Kind:         "cancel",
+		RequestID:    stream.RequestID,
+		CancelReason: "[canceled] Superseded by newer request",
+	}
+	if err := service.handleCancelIntent(cancel); err != nil {
+		t.Fatalf("first handleCancelIntent() error = %v", err)
+	}
+	stream.mu.Lock()
+	stream.ProviderAccumulatedText = "late duplicate fragment"
+	stream.mu.Unlock()
+	if err := service.handleCancelIntent(cancel); err != nil {
+		t.Fatalf("duplicate handleCancelIntent() error = %v", err)
+	}
+
+	persisted, err := service.store.LoadConversation(stream.ConversationID)
+	if err != nil {
+		t.Fatalf("LoadConversation() error = %v", err)
+	}
+	assistantEntries := 0
+	cancelEntries := 0
+	for _, entry := range persisted.Entries {
+		if entry.Kind == "metadata" {
+			var payload metadataPayload
+			if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+				t.Fatalf("decode metadata entry: %v", err)
+			}
+			if payload.Type == "control" && readStringValue(payload.Value["status"]) == "canceled" {
+				cancelEntries++
+			}
+		}
+		if entry.Kind != "assistant_text" {
+			continue
+		}
+		var payload assistantTextPayload
+		if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+			t.Fatalf("decode assistant entry: %v", err)
+		}
+		if payload.Text == "partial answer" {
+			assistantEntries++
+		}
+	}
+	if assistantEntries != 1 {
+		t.Fatalf("persisted interrupted assistant entries = %d, want 1", assistantEntries)
+	}
+	if cancelEntries != 1 {
+		t.Fatalf("persisted cancel metadata entries = %d, want 1", cancelEntries)
+	}
+
+	replay, err := service.projector.ProjectPromptReplay(persisted)
+	if err != nil {
+		t.Fatalf("ProjectPromptReplay() error = %v", err)
+	}
+	found := false
+	for _, message := range replay {
+		if message.Role == "assistant" && strings.TrimSpace(message.Content) == "partial answer" && strings.TrimSpace(message.ReasoningContent) == "partial reasoning" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("replay = %#v, want interrupted assistant output", replay)
+	}
+	checkpoint, err := service.projector.ProjectCheckpointProjection(persisted)
+	if err != nil {
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
+	}
+	if checkpoint == nil || checkpoint.State == nil || len(checkpoint.State.GetTurns()) != 1 {
+		t.Fatalf("checkpoint state = %#v, want interrupted turn", checkpoint)
+	}
+}
+
+func TestCancelPreservesPersistedTurnActivityWithoutLiveAccumulator(t *testing.T) {
+	service, stream, _ := testCheckpointBlobProjection(t)
+	conversation, _, _, err := service.snapshotCheckpointConversation(stream)
+	if err != nil {
+		t.Fatalf("snapshotCheckpointConversation() error = %v", err)
+	}
+	if _, err := service.store.SaveConversationWithEntries(stream.ConversationID, conversation, conversation.Entries); err != nil {
+		t.Fatalf("SaveConversationWithEntries() error = %v", err)
+	}
+
+	if err := service.handleCancelIntent(InboundIntent{
+		Kind:         "cancel",
+		RequestID:    stream.RequestID,
+		CancelReason: "new_message_submitted",
+	}); err != nil {
+		t.Fatalf("handleCancelIntent() error = %v", err)
+	}
+
+	persisted, err := service.store.LoadConversation(stream.ConversationID)
+	if err != nil {
+		t.Fatalf("LoadConversation() error = %v", err)
+	}
+	replay, err := service.projector.ProjectPromptReplay(persisted)
+	if err != nil {
+		t.Fatalf("ProjectPromptReplay() error = %v", err)
+	}
+	for _, message := range replay {
+		if message.Role == "assistant" && strings.TrimSpace(message.Content) == "hi" {
+			return
+		}
+	}
+	t.Fatalf("replay = %#v, want persisted assistant activity", replay)
+}
+
+func TestProjectPromptReplayPreservesLegacyCanceledTurnActivity(t *testing.T) {
+	cancelEntry := newMetadataEntry(1, "request-1", "control", map[string]any{
+		"status":        "canceled",
+		"reason":        "new_message_submitted",
+		"replay_policy": cancelReplayPolicyKeepStableInput,
+	})
+	conversation := &ConversationFile{
+		ConversationID: "conversation-1",
+		NextTurnSeq:    2,
+		Entries: []HistoryEntry{
+			newAssistantTextEntry(1, "request-1", "persisted activity", "", ""),
+			cancelEntry,
+		},
+	}
+
+	replay, err := NewHistoryProjector().ProjectPromptReplay(conversation)
+	if err != nil {
+		t.Fatalf("ProjectPromptReplay() error = %v", err)
+	}
+	for _, message := range replay {
+		if message.Role == "assistant" && strings.TrimSpace(message.Content) == "persisted activity" {
+			return
+		}
+	}
+	t.Fatalf("replay = %#v, want legacy canceled activity", replay)
+}

internal/backend/forwarder/projector.go

@@ -355,6 +355,7 @@ const (
 	cancelReplayPolicyDropTurn        = "drop_turn"
 	cancelReplayPolicyDropUnstarted   = "drop_unstarted_turn"
 	cancelReplayPolicyKeepStableInput = "keep_stable_input"
+	cancelReplayPolicyKeepInterrupted = "keep_interrupted_output"
 )
 
 func sanitizeCanceledReplayEntries(entries []HistoryEntry) []HistoryEntry {
@@ -370,13 +371,19 @@ func sanitizeCanceledReplayEntries(entries []HistoryEntry) []HistoryEntry {
 	for _, entry := range entries {
 		if entry.TurnSeq > 0 {
 			if policy, canceled := canceledTurns[entry.TurnSeq]; canceled {
-				if policy == cancelReplayPolicyDropUnstarted {
+				if policy == cancelReplayPolicyKeepInterrupted {
+					filtered = append(filtered, entry)
+					continue
+				}
+				if policy != cancelReplayPolicyDropTurn {
 					if _, active := activeCanceledTurns[entry.TurnSeq]; active {
-						policy = cancelReplayPolicyKeepStableInput
-					} else {
-						policy = cancelReplayPolicyDropTurn
+						filtered = append(filtered, entry)
+						continue
 					}
 				}
+				if policy == cancelReplayPolicyDropUnstarted {
+					policy = cancelReplayPolicyDropTurn
+				}
 				if policy == cancelReplayPolicyDropTurn || !isStableCanceledTurnInputEntry(entry) {
 					continue
 				}
@@ -450,6 +457,8 @@ func normalizeCancelReplayPolicy(policy string, reason string) string {
 		return cancelReplayPolicyDropUnstarted
 	case cancelReplayPolicyKeepStableInput:
 		return cancelReplayPolicyKeepStableInput
+	case cancelReplayPolicyKeepInterrupted:
+		return cancelReplayPolicyKeepInterrupted
 	default:
 		return cancelReplayPolicyForReason(reason)
 	}
@@ -616,9 +625,13 @@ func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpoin
 	turnIDs := make([][]byte, 0, len(order))
 	for _, turnSeq := range order {
 		entries := grouped[turnSeq]
+		completedToolCalls, err := collectCheckpointCompletedToolCalls(entries)
+		if err != nil {
+			return nil, err
+		}
 		var userMessageID []byte
 		var turnRequestID string
-		stepIDs := make([][]byte, 0, len(entries))
+		steps := make([]*agentv1.ConversationStep, 0, len(entries))
 		seenToolCalls := make(map[string]struct{})
 		openToolCalls := make(map[string]struct{})
 		for _, entry := range entries {
@@ -645,59 +658,48 @@ func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpoin
 					continue
 				}
 				if strings.TrimSpace(payload.ReasoningContent) != "" {
-					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					steps = append(steps, &agentv1.ConversationStep{
 						Message: &agentv1.ConversationStep_ThinkingMessage{
 							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
 						},
 					})
-					if err != nil {
-						return nil, err
-					}
-					stepIDs = append(stepIDs, stepID)
 				}
 				if strings.TrimSpace(payload.Text) == "" {
 					continue
 				}
-				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+				steps = append(steps, &agentv1.ConversationStep{
 					Message: &agentv1.ConversationStep_AssistantMessage{
 						AssistantMessage: &agentv1.AssistantMessage{Text: strings.TrimSpace(payload.Text)},
 					},
 				})
-				if err != nil {
-					return nil, err
-				}
-				stepIDs = append(stepIDs, stepID)
 			case "tool_call":
 				var payload toolCallEntryPayload
 				if err := json.Unmarshal(entry.Payload, &payload); err != nil {
 					return nil, err
 				}
 				if strings.TrimSpace(payload.ReasoningContent) != "" {
-					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					steps = append(steps, &agentv1.ConversationStep{
 						Message: &agentv1.ConversationStep_ThinkingMessage{
 							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
 						},
 					})
-					if err != nil {
-						return nil, err
-					}
-					stepIDs = append(stepIDs, stepID)
 				}
 				toolCall := &agentv1.ToolCall{}
+				toolCallID := strings.TrimSpace(payload.ToolCallID)
 				if err := protojson.Unmarshal(payload.ToolCall, toolCall); err != nil {
 					return nil, err
 				}
-				if !shouldPersistToolResultName(firstNonEmpty(strings.TrimSpace(payload.ToolName), inferToolName(toolCall))) {
-					continue
+				if completedPayload := completedToolCalls[toolCallID]; len(completedPayload) > 0 {
+					completedToolCall := &agentv1.ToolCall{}
+					if err := protojson.Unmarshal(completedPayload, completedToolCall); err != nil {
+						return nil, err
+					}
+					proto.Merge(toolCall, completedToolCall)
 				}
-				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+				steps = append(steps, &agentv1.ConversationStep{
 					Message: &agentv1.ConversationStep_ToolCall{ToolCall: toolCall},
 				})
-				if err != nil {
-					return nil, err
-				}
-				stepIDs = append(stepIDs, stepID)
-				if toolCallID := strings.TrimSpace(payload.ToolCallID); toolCallID != "" {
+				if toolCallID != "" {
 					seenToolCalls[toolCallID] = struct{}{}
 					openToolCalls[toolCallID] = struct{}{}
 				}
@@ -706,22 +708,19 @@ func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpoin
 				if err := json.Unmarshal(entry.Payload, &payload); err != nil {
 					return nil, err
 				}
-				if toolCallID := strings.TrimSpace(payload.ToolCallID); toolCallID != "" {
-					if _, ok := seenToolCalls[toolCallID]; ok {
-						delete(openToolCalls, toolCallID)
-						continue
-					}
+				toolCallID := strings.TrimSpace(payload.ToolCallID)
+				if toolCallID != "" {
+					delete(openToolCalls, toolCallID)
+				}
+				if _, ok := seenToolCalls[toolCallID]; ok {
+					continue
 				}
 				if strings.TrimSpace(payload.ReasoningContent) != "" {
-					stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+					steps = append(steps, &agentv1.ConversationStep{
 						Message: &agentv1.ConversationStep_ThinkingMessage{
 							ThinkingMessage: &agentv1.ThinkingMessage{Text: payload.ReasoningContent},
 						},
 					})
-					if err != nil {
-						return nil, err
-					}
-					stepIDs = append(stepIDs, stepID)
 				}
 				if len(payload.ToolCall) == 0 {
 					continue
@@ -730,21 +729,22 @@ func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpoin
 				if err := protojson.Unmarshal(payload.ToolCall, toolCall); err != nil {
 					return nil, err
 				}
-				if !shouldPersistToolResultName(firstNonEmpty(strings.TrimSpace(payload.ToolName), inferToolName(toolCall))) {
-					continue
-				}
-				stepID, err := addCheckpointStepBlob(blobs, &agentv1.ConversationStep{
+				steps = append(steps, &agentv1.ConversationStep{
 					Message: &agentv1.ConversationStep_ToolCall{ToolCall: toolCall},
 				})
-				if err != nil {
-					return nil, err
-				}
-				stepIDs = append(stepIDs, stepID)
 			}
 		}
-		if len(userMessageID) == 0 && len(stepIDs) == 0 {
+		if len(userMessageID) == 0 && len(steps) == 0 {
 			continue
 		}
+		stepIDs := make([][]byte, 0, len(steps))
+		for _, step := range steps {
+			stepID, err := addCheckpointStepBlob(blobs, step)
+			if err != nil {
+				return nil, err
+			}
+			stepIDs = append(stepIDs, stepID)
+		}
 		agentTurn := &agentv1.AgentConversationTurnStructure{
 			UserMessage: userMessageID,
 			Steps:       stepIDs,
@@ -765,6 +765,23 @@ func projectCheckpointTurnBlobs(conversation *ConversationFile, blobs *checkpoin
 	return turnIDs, nil
 }
 
+func collectCheckpointCompletedToolCalls(entries []HistoryEntry) (map[string]json.RawMessage, error) {
+	completed := make(map[string]json.RawMessage)
+	for _, entry := range entries {
+		if strings.TrimSpace(entry.Kind) != "tool_result" {
+			continue
+		}
+		var payload toolResultEntryPayload
+		if err := json.Unmarshal(entry.Payload, &payload); err != nil {
+			return nil, err
+		}
+		if toolCallID := strings.TrimSpace(payload.ToolCallID); toolCallID != "" && len(payload.ToolCall) > 0 {
+			completed[toolCallID] = payload.ToolCall
+		}
+	}
+	return completed, nil
+}
+
 func addCheckpointStepBlob(blobs *checkpointBlobGraph, step *agentv1.ConversationStep) ([]byte, error) {
 	payload, err := proto.Marshal(step)
 	if err != nil {
@@ -1209,7 +1226,7 @@ func trimReplayDanglingAssistantToolCalls(messages []modeladapter.Message) []mod
 	return trimmed
 }
 
-func shouldPersistToolResultName(toolName string) bool {
+func shouldPersistCheckpointReplayToolResultName(toolName string) bool {
 	switch strings.TrimSpace(toolName) {
 	case "PatchEdit", "PatchEditLines", "PatchEditSpan", "Edit", "Write", "GenerateImage":
 		return true
@@ -1218,60 +1235,6 @@ func shouldPersistToolResultName(toolName string) bool {
 	}
 }
 
-func filterCheckpointTurns(rawTurns [][]byte) [][]byte {
-	if len(rawTurns) == 0 {
-		return nil
-	}
-	filtered := make([][]byte, 0, len(rawTurns))
-	for _, rawTurn := range rawTurns {
-		if len(rawTurn) == 0 {
-			continue
-		}
-		turn := &agentv1.ConversationTurnStructure{}
-		if err := proto.Unmarshal(rawTurn, turn); err != nil {
-			filtered = append(filtered, append([]byte(nil), rawTurn...))
-			continue
-		}
-		agentTurn := turn.GetAgentConversationTurn()
-		if agentTurn == nil {
-			filtered = append(filtered, append([]byte(nil), rawTurn...))
-			continue
-		}
-
-		nextSteps := make([][]byte, 0, len(agentTurn.GetSteps()))
-		for _, rawStep := range agentTurn.GetSteps() {
-			if len(rawStep) == 0 {
-				continue
-			}
-			step := &agentv1.ConversationStep{}
-			if err := proto.Unmarshal(rawStep, step); err != nil {
-				continue
-			}
-			if toolCall := step.GetToolCall(); toolCall != nil && !shouldPersistToolResultName(inferToolName(toolCall)) {
-				continue
-			}
-			nextSteps = append(nextSteps, append([]byte(nil), rawStep...))
-		}
-		if len(agentTurn.GetUserMessage()) == 0 && len(nextSteps) == 0 {
-			continue
-		}
-		encoded, err := proto.Marshal(&agentv1.ConversationTurnStructure{
-			Turn: &agentv1.ConversationTurnStructure_AgentConversationTurn{
-				AgentConversationTurn: &agentv1.AgentConversationTurnStructure{
-					UserMessage: append([]byte(nil), agentTurn.GetUserMessage()...),
-					Steps:       nextSteps,
-				},
-			},
-		})
-		if err != nil {
-			filtered = append(filtered, append([]byte(nil), rawTurn...))
-			continue
-		}
-		filtered = append(filtered, encoded)
-	}
-	return filtered
-}
-
 func filterCheckpointPersistentToolReplay(messages []promptengine.Message) []promptengine.Message {
 	if len(messages) == 0 {
 		return nil
@@ -1282,7 +1245,7 @@ func filterCheckpointPersistentToolReplay(messages []promptengine.Message) []pro
 		if strings.TrimSpace(message.Role) == "assistant" && len(message.ToolCalls) > 0 {
 			nextToolCalls := make([]promptengine.ToolCallDescriptor, 0, len(message.ToolCalls))
 			for _, toolCall := range message.ToolCalls {
-				if !shouldPersistToolResultName(toolCall.Function.Name) {
+				if !shouldPersistCheckpointReplayToolResultName(toolCall.Function.Name) {
 					skippedToolCallIDs[strings.TrimSpace(toolCall.ID)] = struct{}{}
 					continue
 				}
@@ -1299,7 +1262,7 @@ func filterCheckpointPersistentToolReplay(messages []promptengine.Message) []pro
 			if _, ok := skippedToolCallIDs[strings.TrimSpace(message.ToolCallID)]; ok {
 				continue
 			}
-			if !shouldPersistToolResultName(message.Name) {
+			if !shouldPersistCheckpointReplayToolResultName(message.Name) {
 				continue
 			}
 		}

internal/backend/forwarder/projector_fork_test.go

@@ -1,7 +1,9 @@
 package forwarder
 
 import (
+	"bytes"
 	"crypto/sha256"
+	"encoding/json"
 	"strings"
 	"testing"
 
@@ -9,6 +11,7 @@ import (
 	"google.golang.org/protobuf/proto"
 
 	"cursor/gen/agentv1"
+	promptengine "cursor/internal/backend/agent/prompt"
 )
 
 func TestProjectCheckpointProjectionBuildsResolvableForkState(t *testing.T) {
@@ -138,3 +141,240 @@ func TestProjectCheckpointProjectionKeepsForkPointIsolatedFromLaterHistory(t *te
 		t.Fatalf("fork snapshots are not isolated: midpoint=%#v latest=%#v", midpointMessages, latestMessages)
 	}
 }
+
+func TestProjectCheckpointProjectionMergesToolCallWithCompletedResult(t *testing.T) {
+	userPayload, err := protojson.Marshal(&agentv1.UserMessage{Text: "inspect file", MessageId: "message-1"})
+	if err != nil {
+		t.Fatalf("marshal user message: %v", err)
+	}
+	startedAt := uint64(100)
+	toolCallID := "call-1"
+	startedToolCall := checkpointTestToolCallPayload(t, &agentv1.ToolCall{
+		ToolCallId:  &toolCallID,
+		StartedAtMs: &startedAt,
+		Tool: &agentv1.ToolCall_ReadToolCall{
+			ReadToolCall: &agentv1.ReadToolCall{
+				Args: &agentv1.ReadToolArgs{Path: "/tmp/example.txt"},
+			},
+		},
+	})
+	completedAt := uint64(200)
+	completedToolCall := checkpointTestToolCallPayload(t, &agentv1.ToolCall{
+		CompletedAtMs: &completedAt,
+		Tool: &agentv1.ToolCall_ReadToolCall{
+			ReadToolCall: &agentv1.ReadToolCall{
+				Result: &agentv1.ReadToolResult{
+					Result: &agentv1.ReadToolResult_Success{
+						Success: &agentv1.ReadToolSuccess{
+							Path:       "/tmp/example.txt",
+							TotalLines: 1,
+							Output:     &agentv1.ReadToolSuccess_Content{Content: "file contents"},
+						},
+					},
+				},
+			},
+		},
+	})
+	conversation := &ConversationFile{
+		ConversationID: "conversation-1",
+		Mode:           "agent",
+		NextTurnSeq:    2,
+		Entries: []HistoryEntry{
+			{Seq: 1, TurnSeq: 1, RequestID: "request-1", Role: "user", Kind: "user_message", Payload: userPayload},
+			newAssistantTextEntry(1, "request-1", "before", "", ""),
+			newToolCallEntry(1, "request-1", "call-1", "Read", "", "", startedToolCall),
+			newToolResultEntry(1, "request-1", "call-1", "Read", `{"path":"/tmp/example.txt"}`, "file contents", "", completedToolCall),
+			newAssistantTextEntry(1, "request-1", "after", "", ""),
+		},
+	}
+
+	projection, err := NewHistoryProjector().ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
+	}
+	if len(projection.Blobs) != 5 {
+		t.Fatalf("checkpoint blobs = %d, want user, three final steps, and turn", len(projection.Blobs))
+	}
+	steps := checkpointProjectionSteps(t, projection)
+	if len(steps) != 3 {
+		t.Fatalf("checkpoint steps = %d, want assistant, completed Read, assistant", len(steps))
+	}
+	if steps[0].GetAssistantMessage().GetText() != "before" || steps[2].GetAssistantMessage().GetText() != "after" {
+		t.Fatalf("checkpoint step ordering changed: %#v", steps)
+	}
+	mergedToolCall := steps[1].GetToolCall()
+	readCall := mergedToolCall.GetReadToolCall()
+	if readCall == nil || readCall.GetResult().GetSuccess().GetContent() != "file contents" {
+		t.Fatalf("checkpoint Read step does not contain completed result: %#v", steps[1].GetToolCall())
+	}
+	if readCall.GetArgs().GetPath() != "/tmp/example.txt" || mergedToolCall.GetToolCallId() != toolCallID || mergedToolCall.GetStartedAtMs() != startedAt || mergedToolCall.GetCompletedAtMs() != completedAt {
+		t.Fatalf("checkpoint Read step lost started-call fields: %#v", mergedToolCall)
+	}
+
+	replay, err := promptengine.DecodeReplayMessages(projection.State.GetRootPromptMessagesJson())
+	if err != nil {
+		t.Fatalf("decode root prompt replay: %v", err)
+	}
+	for _, message := range replay {
+		if message.Name == "Read" || len(message.ToolCalls) > 0 {
+			t.Fatalf("UI-only Read result leaked into root prompt replay: %#v", replay)
+		}
+	}
+}
+
+func TestProjectCheckpointProjectionIsIdempotentAndDoesNotMutateHistory(t *testing.T) {
+	userPayload, err := protojson.Marshal(&agentv1.UserMessage{Text: "inspect file", MessageId: "message-1"})
+	if err != nil {
+		t.Fatalf("marshal user message: %v", err)
+	}
+	completedToolCall := checkpointTestReadToolCall(t, &agentv1.ReadToolResult{
+		Result: &agentv1.ReadToolResult_Success{
+			Success: &agentv1.ReadToolSuccess{
+				Path:       "/tmp/example.txt",
+				TotalLines: 1,
+				Output:     &agentv1.ReadToolSuccess_Content{Content: "file contents"},
+			},
+		},
+	})
+	conversation := &ConversationFile{
+		ConversationID: "conversation-1",
+		Mode:           "agent",
+		NextTurnSeq:    2,
+		Entries: []HistoryEntry{
+			{Seq: 1, TurnSeq: 1, RequestID: "request-1", Role: "user", Kind: "user_message", Payload: userPayload},
+			newToolCallEntry(1, "request-1", "call-1", "Read", "", "", checkpointTestReadToolCall(t, nil)),
+			newToolResultEntry(1, "request-1", "call-1", "Read", `{"path":"/tmp/example.txt"}`, "file contents", "", completedToolCall),
+		},
+	}
+	before, err := json.Marshal(conversation)
+	if err != nil {
+		t.Fatalf("marshal conversation before projection: %v", err)
+	}
+
+	projector := NewHistoryProjector()
+	first, err := projector.ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("first projection: %v", err)
+	}
+	second, err := projector.ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("second projection: %v", err)
+	}
+
+	if !proto.Equal(first.State, second.State) {
+		t.Fatalf("repeated projection changed checkpoint state: first=%#v second=%#v", first.State, second.State)
+	}
+	if len(first.Blobs) != len(second.Blobs) {
+		t.Fatalf("repeated projection changed blob count: first=%d second=%d", len(first.Blobs), len(second.Blobs))
+	}
+	for index := range first.Blobs {
+		if !bytes.Equal(first.Blobs[index].ID, second.Blobs[index].ID) || !bytes.Equal(first.Blobs[index].Data, second.Blobs[index].Data) {
+			t.Fatalf("repeated projection changed blob %d", index)
+		}
+	}
+	after, err := json.Marshal(conversation)
+	if err != nil {
+		t.Fatalf("marshal conversation after projection: %v", err)
+	}
+	if !bytes.Equal(before, after) {
+		t.Fatalf("checkpoint projection mutated semantic history:\nbefore=%s\nafter=%s", before, after)
+	}
+}
+
+func TestProjectCheckpointProjectionKeepsStartedToolCallWhenResultPayloadIsMissing(t *testing.T) {
+	startedToolCall := checkpointTestReadToolCall(t, nil)
+	conversation := &ConversationFile{
+		ConversationID: "conversation-1",
+		Mode:           "agent",
+		NextTurnSeq:    2,
+		Entries: []HistoryEntry{
+			newToolCallEntry(1, "request-1", "call-1", "Read", "", "", startedToolCall),
+			newToolResultEntry(1, "request-1", "call-1", "Read", `{"path":"/tmp/example.txt"}`, "read failed", "", nil),
+		},
+	}
+
+	projection, err := NewHistoryProjector().ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
+	}
+	steps := checkpointProjectionSteps(t, projection)
+	if len(steps) != 1 {
+		t.Fatalf("checkpoint steps = %d, want the original Read step", len(steps))
+	}
+	readCall := steps[0].GetToolCall().GetReadToolCall()
+	if readCall == nil || readCall.GetArgs().GetPath() != "/tmp/example.txt" || readCall.GetResult() != nil {
+		t.Fatalf("checkpoint did not preserve the original Read call: %#v", steps[0].GetToolCall())
+	}
+}
+
+func TestProjectCheckpointProjectionAppendsLegacyResultWithoutToolCallEntry(t *testing.T) {
+	completedToolCall := checkpointTestReadToolCall(t, &agentv1.ReadToolResult{
+		Result: &agentv1.ReadToolResult_Error{Error: &agentv1.ReadToolError{ErrorMessage: "not readable"}},
+	})
+	conversation := &ConversationFile{
+		ConversationID: "conversation-1",
+		Mode:           "agent",
+		NextTurnSeq:    2,
+		Entries: []HistoryEntry{
+			newToolResultEntry(1, "request-1", "call-1", "Read", `{"path":"/tmp/example.txt"}`, "not readable", "", completedToolCall),
+		},
+	}
+
+	projection, err := NewHistoryProjector().ProjectCheckpointProjection(conversation)
+	if err != nil {
+		t.Fatalf("ProjectCheckpointProjection() error = %v", err)
+	}
+	steps := checkpointProjectionSteps(t, projection)
+	if len(steps) != 1 || steps[0].GetToolCall().GetReadToolCall().GetResult().GetError().GetErrorMessage() != "not readable" {
+		t.Fatalf("legacy result-only Read step was not preserved: %#v", steps)
+	}
+}
+
+func checkpointTestReadToolCall(t *testing.T, result *agentv1.ReadToolResult) []byte {
+	t.Helper()
+	return checkpointTestToolCallPayload(t, &agentv1.ToolCall{
+		Tool: &agentv1.ToolCall_ReadToolCall{
+			ReadToolCall: &agentv1.ReadToolCall{
+				Args:   &agentv1.ReadToolArgs{Path: "/tmp/example.txt"},
+				Result: result,
+			},
+		},
+	})
+}
+
+func checkpointTestToolCallPayload(t *testing.T, toolCall *agentv1.ToolCall) []byte {
+	t.Helper()
+	payload, err := protojson.Marshal(toolCall)
+	if err != nil {
+		t.Fatalf("marshal Read tool call: %v", err)
+	}
+	return payload
+}
+
+func checkpointProjectionSteps(t *testing.T, projection *CheckpointProjection) []*agentv1.ConversationStep {
+	t.Helper()
+	if projection == nil || projection.State == nil || len(projection.State.GetTurns()) != 1 {
+		t.Fatalf("checkpoint turns = %#v, want exactly one turn", projection)
+	}
+	blobs := make(map[string][]byte, len(projection.Blobs))
+	for _, blob := range projection.Blobs {
+		blobs[string(blob.ID)] = blob.Data
+	}
+	turn := &agentv1.ConversationTurnStructure{}
+	if err := proto.Unmarshal(blobs[string(projection.State.GetTurns()[0])], turn); err != nil {
+		t.Fatalf("decode checkpoint turn: %v", err)
+	}
+	agentTurn := turn.GetAgentConversationTurn()
+	if agentTurn == nil {
+		t.Fatal("checkpoint turn does not contain an agent turn")
+	}
+	steps := make([]*agentv1.ConversationStep, 0, len(agentTurn.GetSteps()))
+	for _, stepID := range agentTurn.GetSteps() {
+		step := &agentv1.ConversationStep{}
+		if err := proto.Unmarshal(blobs[string(stepID)], step); err != nil {
+			t.Fatalf("decode checkpoint step: %v", err)
+		}
+		steps = append(steps, step)
+	}
+	return steps
+}

internal/backend/forwarder/service.go

@@ -3,6 +3,8 @@ package forwarder
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -834,14 +836,22 @@ func (service *Service) handleCancelIntent(intent InboundIntent) error {
 	}
 	hasCheckpoint := checkpointConversationInitialized(stream)
 	if hasCheckpoint {
+		preservedInterruptedOutput, err := service.persistInterruptedProviderOutput(stream)
+		if err != nil {
+			return err
+		}
 		cancelReason := firstNonEmpty(intent.CancelReason, "user aborted")
-		_, err := service.appendConversationEntries(stream, stream.ConversationID, []HistoryEntry{
-			newMetadataEntry(stream.TurnSeq, intent.RequestID, "control", map[string]any{
-				"status":        "canceled",
-				"reason":        cancelReason,
-				"replay_policy": cancelReplayPolicyForReason(cancelReason),
-			}),
+		replayPolicy := cancelReplayPolicyForReason(cancelReason)
+		if preservedInterruptedOutput || checkpointTurnHasReplayActivity(stream) {
+			replayPolicy = cancelReplayPolicyKeepInterrupted
+		}
+		cancelEntry := newMetadataEntry(stream.TurnSeq, intent.RequestID, "control", map[string]any{
+			"status":        "canceled",
+			"reason":        cancelReason,
+			"replay_policy": replayPolicy,
 		})
+		cancelEntry.IdempotencyKey = cancelMetadataIdempotencyKey(stream.TurnSeq, intent.RequestID)
+		_, err = service.appendConversationEntries(stream, stream.ConversationID, []HistoryEntry{cancelEntry})
 		if err != nil {
 			return err
 		}
@@ -869,6 +879,89 @@ func (service *Service) handleCancelIntent(intent InboundIntent) error {
 	return service.broker.Cancel(intent.RequestID, firstNonEmpty(intent.CancelReason, "[canceled] User aborted request"))
 }
 
+func checkpointTurnHasReplayActivity(stream *ActiveStream) bool {
+	if stream == nil {
+		return false
+	}
+	stream.mu.Lock()
+	defer stream.mu.Unlock()
+	if stream.CheckpointConversation == nil {
+		return false
+	}
+	for _, entry := range stream.CheckpointConversation.Entries {
+		if entry.TurnSeq == stream.TurnSeq && isCanceledTurnActivityEntry(entry) {
+			return true
+		}
+	}
+	return false
+}
+
+// persistInterruptedProviderOutput commits the current provider pass before cancellation.
+// The entry key is stable for this provider pass, so repeated cancellation handling is a no-op.
+func (service *Service) persistInterruptedProviderOutput(stream *ActiveStream) (bool, error) {
+	if stream == nil {
+		return false, nil
+	}
+	stream.mu.Lock()
+	turnSeq := stream.TurnSeq
+	requestID := strings.TrimSpace(stream.RequestID)
+	modelCallID := strings.TrimSpace(stream.CurrentModelCallID)
+	providerPass := stream.ProviderPassCount
+	text := stream.ProviderAccumulatedText
+	reasoning := stream.ProviderAccumulatedReasoning
+	reasoningSignature := stream.ProviderAccumulatedReasoningSignature
+	reasoningSignatureSource := stream.ProviderAccumulatedReasoningSignatureSource
+	reasoningItemID := stream.ProviderAccumulatedReasoningItemID
+	reasoningStatus := stream.ProviderAccumulatedReasoningStatus
+	reasoningSummary := append([]byte(nil), stream.ProviderAccumulatedReasoningSummary...)
+	stream.mu.Unlock()
+	if strings.TrimSpace(text) == "" && !hasReplayableReasoningPayload(reasoning, reasoningSignature, reasoningSignatureSource) {
+		return false, nil
+	}
+	key := interruptedProviderOutputIdempotencyKey(turnSeq, requestID, modelCallID, providerPass)
+	_, err := service.appendConversationEntries(stream, stream.ConversationID, []HistoryEntry{
+		{
+			TurnSeq:        turnSeq,
+			RequestID:      requestID,
+			IdempotencyKey: key,
+			Role:           "assistant",
+			Kind:           "assistant_text",
+			Payload: newAssistantTextPayload(
+				text,
+				reasoning,
+				reasoningSignature,
+				reasoningSignatureSource,
+				reasoningItemID,
+				reasoningStatus,
+				reasoningSummary,
+			),
+		},
+	})
+	return true, err
+}
+
+func interruptedProviderOutputIdempotencyKey(turnSeq int64, requestID string, modelCallID string, providerPass int) string {
+	payload := strings.Join([]string{
+		"provider_interrupted_output",
+		fmt.Sprintf("%d", turnSeq),
+		strings.TrimSpace(requestID),
+		strings.TrimSpace(modelCallID),
+		fmt.Sprintf("%d", providerPass),
+	}, "\x00")
+	digest := sha256.Sum256([]byte(payload))
+	return "provider-interrupted-output:" + hex.EncodeToString(digest[:])
+}
+
+func cancelMetadataIdempotencyKey(turnSeq int64, requestID string) string {
+	payload := strings.Join([]string{
+		"cancel",
+		fmt.Sprintf("%d", turnSeq),
+		strings.TrimSpace(requestID),
+	}, "\x00")
+	digest := sha256.Sum256([]byte(payload))
+	return "cancel:" + hex.EncodeToString(digest[:])
+}
+
 // handleExecResult 处理客户端返回的执行桥结果，并在终态时把 tool_result 写回 history。
 func (service *Service) handleExecResult(intent InboundIntent) error {
 	stream, ok := service.broker.Get(intent.RequestID)
@@ -2431,6 +2524,16 @@ func newAssistantTextEntry(turnSeq int64, requestID string, text string, reasoni
 }
 
 func newAssistantTextEntryWithProviderMetadata(turnSeq int64, requestID string, text string, reasoningContent string, reasoningSignature string, reasoningSignatureSource string, reasoningItemID string, reasoningStatus string, reasoningSummary json.RawMessage) HistoryEntry {
+	return HistoryEntry{
+		TurnSeq:   turnSeq,
+		RequestID: strings.TrimSpace(requestID),
+		Role:      "assistant",
+		Kind:      "assistant_text",
+		Payload:   newAssistantTextPayload(text, reasoningContent, reasoningSignature, reasoningSignatureSource, reasoningItemID, reasoningStatus, reasoningSummary),
+	}
+}
+
+func newAssistantTextPayload(text string, reasoningContent string, reasoningSignature string, reasoningSignatureSource string, reasoningItemID string, reasoningStatus string, reasoningSummary json.RawMessage) json.RawMessage {
 	payload, _ := json.Marshal(assistantTextPayload{
 		Text:                     text,
 		ReasoningContent:         reasoningContent,
@@ -2440,13 +2543,7 @@ func newAssistantTextEntryWithProviderMetadata(turnSeq int64, requestID string,
 		ReasoningStatus:          strings.TrimSpace(reasoningStatus),
 		ReasoningSummary:         append(json.RawMessage(nil), reasoningSummary...),
 	})
-	return HistoryEntry{
-		TurnSeq:   turnSeq,
-		RequestID: strings.TrimSpace(requestID),
-		Role:      "assistant",
-		Kind:      "assistant_text",
-		Payload:   payload,
-	}
+	return payload
 }
 
 // newToolCallEntry 构造 tool_call entry。

internal/backend/forwarder/types.go

@@ -79,6 +79,7 @@ type HistoryEntry struct {
 	Seq              int64           `json:"seq"`
 	TurnSeq          int64           `json:"turn_seq"`
 	RequestID        string          `json:"request_id,omitempty"`
+	IdempotencyKey   string          `json:"idempotency_key,omitempty"`
 	Role             string          `json:"role"`
 	Kind             string          `json:"kind"`
 	ToolCallID       string          `json:"tool_call_id,omitempty"`